Time Trial Aerodynamics

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(Originally written in 2009)

Bike Alone_d

My first time trial was nine years ago at a Fiesta Island TT. I rode a Specialized M4 road bike with Spinergy Rev-X wheels and clip on aerobars. Coming from a cross-country mountain bike background, and having dabbled in multisport events the previous year, I assumed that racing and training for a time trial would be “easy” compared to MTB events. I quickly learned that the sport of time trial racing is highly technical, and it can take years to develop optimal fitness and the skills needed to excel in this discipline. In the following article, I will discuss some of the finer points of basic time trial aerodynamics I have picked up along the way, and the priorities one should consider when upgrading equipment.

I often get asked the question about where to invest upgrade dollars when considering new equipment and gear. From results others have shared with me, my time in the wind tunnel observing athletes, and the various data reports that I have read, I will offer the following list of priorities for a time trial bicycle and the rider. This evaluation is fully debatable and represents only my opinion. The results below were gained from various sources and in most cases represent a rider at 30MPH at zero degree yaw over a 40K course.

** Maximum time savings for these upgrades over 40K:

Handlebars (Reference: J Cobb, Vision Tech)

From: To: Savings:
Drop Bars Integrated Aerobars 2 – 4 Minutes
Drops/clip-ons Integrated Aerobars 15 secs – 1 Minute

Clothing (Reference: Rainer Pivit)

From: To: Savings:
Shorts & Jersey Skin Suit 29 seconds
— Add Shoe Covers 13 Seconds

Wheels (Reference: J Cobb)

From: To: Savings:
“Box Style” 32 spoke wheels Deep front/disc rear 2 – 3 Minutes
Deep front/rear Deep front/disc rear 30 Seconds

Frame (Reference: J Cobb, Martin/Cervelo)

From: To: Savings:
Round tubing Airfoil chainstays, down/seat tube & post 30 secs – 2 Minutes

Fork (Reference: Oval Concepts, J Cobb, D Bunce)

From: To: Savings:
Round tube fork Oval Jetstream 35 seconds
Round tube fork Ouzo Pro Aero 30 seconds
Round tube fork Hotta 30 seconds

Helmet (Reference: J Cobb)

From: To: Savings:
Standard road helmet L.G. Prologue 30 seconds
Standard road helmet Standard road helmet, tape over vents 10 – 30 seconds

Aero Wheels – Using aero wheels (vs. 32 spoke box rim wheels) can save you two minutes over a 40K time trial. The fastest wheels are a deep dish front (Zipp 1080, Zipp 808, Stinger 9, HED H3) paired with a disc rear (Zipp, Bontrager, Mavic, HED). Aero wheels definitely make a substantial difference in reducing aerodynamic drag. When deciding which wheels to purchase, HED makes a yaw calculator and Zipp has aero test results to help you in your decision.

Aerobars – Aerobars are the #1 most important feature of any time trial bicycle. Integrated one-piece aerobars are more aerodynamic than clip on bars. Wind tunnel tests show that using aerobars can save 4 minutes over a 40K time trial course. Currently there is an abundance of aerobars on the market, and the fastest aerobars carry the labels of Profile Design, Zipp, Bontrager, HED and Vision Tech. These new one-piece units are some of the best designed and likely the most aerodynamic aerobars made today. For clip on aerobars, the classic Syntace C2 (a triathlon favorite) has been moved aside by Zipp and Vision Tech base bar and clip on bar systems.

Aero Frames – Many frames on the market claim to be aero. The most important feature that a frame can offer you is the fit, followed by aerodynamics, stiffness and comfort. Many new aero frames are designed around UCI rules. This requires a double-diamond design, and 3:1 tubing aspect ratio. If racing national championship events you should consider a UCI legal frameset. Frame aerodynamics are important, however fit is even more so. When selecting a frame, fit and comfort are paramount considerations. How do you know if the frame you are considering is aero? Start with the tubeshapes of the bicycle. Are the tubes airfoil shaped? Is the downtube narrow and deep? Is there a closely spaced rear wheel cut-out? Aero seatpost? These are some items to consider. Some of the top companies today that produce aero time trial frames are (in no particular order) Specialized, Cervelo, Trek, Orbea, Kestrel, Felt, Softride, Corima, Lotus, Elite, Quintana Roo, Aegis, Pinarello and Colnago.

Conclusion – Aerodynamic gear does not come cheap, so keep these two points in mind when considering upgrading your equipment:

  • Buy the fastest gear you can afford, but remember that position on the bike is paramount to anything you bolt onto it
  • Buy an aero helmet first, then a skinsuit, then upgrade your wheels, then the frame

Aero Helmet Wind Tunnel Testing

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We tested several aero helmets. Not surprisingly, we found that every athlete responds differently to aero equipment. One common theme is that there were “good” and “bad” helmets, regardless of the rider that was testing them. Results for the Louis Garneau and Giro are shown above. Surprisingly, the old Troxel titanium dimpled helmet also tested well.
  • Lower profile helmets are more aerodynamic
  • Shape matters, the helmet should transition to the rider’s back as seamlessly as possible
  • Visors were not tested – may or may not have an impact
  • “Filled” tails were not tested – again, may have an impact especially for tails that sit up off the back
We also tested three hand positions, the “pointer”, “pistol”, and “superman”. What was apparent is that there were little discernible differences in the results, with the best result from the original baseline “pointer” position. This is likely due to the smaller frontal area that the position provides, as compared to the other two positions.
  • All three hand positions are fairly close in terms of aerodynamics
  • Gloves were not tested, however if gloves were to be worn, a low profile set such as the aero versions from Castelli or Nike should be considered
  • Smaller hand profiles are important

How to Calculate Cycling Aerodynamic Drag Using a Power Meter

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Originally seen at http://cyclingillustrated.com/how-to-calculate-cycling-aerodynamic-drag-using-a-power-meter/

Don’t have access to a wind tunnel or velodrome? No problem. You can test your own time trial position or gear/helmet changes using the following instructions. All you need is a calm day, an uninterrupted flat course, a power meter, and the following instructions. Using details in the screenshot image above, and the calculator provided by Analytic Cycling, you can estimate the power required to complete a time trial for any specific time. In the example screen shot, I used a 20k time trial at sea level on a calm day. An alternative method for estimating aerodynamic drag can be found by measuring anthropometric data.

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The calculators and links required are located at:

For this example, rolling resistance for the course is in the numbers in the screen shot. For purposes of the estimate, use the defaults in the image for pedal range, wind, drag, air pressure, Crr, and rise.

Then plug in your crank length and average cadence.

Also plug in the speed in meters per second, then change the frontal area up or down until the power matches the average from the race, with the caveat of the time lost to inertia at the start (add 5 seconds to the time).

An example for a hypothetical “aero-tuned” cyclist competing at the San Diego Time Trial at Fiesta Island, which is a point to point 3-lap 20k course:

  • .44 frontal area [.440m^2 x 0.5 = .220m^2 of CdA]
  • 83kg rider and bike Slope – 0.0015, crr 0.004, 12.69 m/s – 332w – 26:15 for 20k
  • + 5 secs first lap = 26:20

  • .44 frontal area [.440m^2 x 0.5 = .220m^2 of CdA]
  • 83kg rider and bike Slope – 0.0015, crr 0.004, 12.82 m/s – 341w – 26:00 for 20k
  • + 5 secs first lap = 26:05

Once you know your CdA, you can calculate your Functional Threshold Power (FTP) as it relates to your aerodynamic drag (CdA) in terms of W/m^2. With this number you can then compare your aerodynamic drag/power ratio to other riders.